This invention relates to tuning apparatus for a frequency synthesiser tuner and, more particularly for such tuner used in television receiver or video cassette recorder or video satellite receiver or consumer set top box.
As the input stage to a television receiver, the tuner provides the primary role of selecting the desired signal and excluding all others. The tuner typically provides a 9-Mhz passband for the 6-Mhz channel. This passband is usually achieved by four tuned circuits: a single-tuned input filter located between the antenna input and the RF amplifier, a double-tuned band pass filter located between the RF amplifier and mixer stages, and a single-tuned intermediate frequency (IF) circuit at the output of the mixer. The first three are frequency-selective to the desired channel by varying their inductance, capacitance, or both. The partition of the VHF or UHF or hyperband, band between those stages is not standard and varies according to each maker of tuner. The mixer output is always tuned to the IF frequency of 45;75 or 38.9 Mhz according to the standard of the received broadcast signal.
The selectivity provided by the RF stage is to reduce or eliminate undesired signals from the desired channel, e.g., image, thereby eliminating or reducing interference problems.
In a typical tuner, such as tuner for receiving broadcast television signals which are transmitted over respective broadcast frequencies, the tuning condition of the tuner is determined by the frequency of the local oscillating signal which is mixed with the received broadcast frequencies to produce a predetermined intermediate frequency (IF) signal. The IF signal is then demodulated to get the video and audio broadcast signals. As the local oscillating frequency is changed, the tuner is tuned to different broadcast frequencies to receive the program information which is broadcast thereover. Typically, a local oscillator includes a variable capacitor or inductance which, when value thereof varies, the local oscillating frequency correspondingly varies. By adjusting for instance, an inductance coil, an operator may change the local oscillating frequency as desired and, thus, may establish any desired tuning condition of the tuner.
The variable capacitor is generally constructed as a variable capacitance diode, whose capacitance value is determined by a control tuning voltage (vt) applied thereto. Since the same control voltage will result in the same tuning condition, tuners are known in which digital techniques are relied upon for storing digital representations of respective control voltages, which digital representations can be retrieved, as desired, so as to establish a capacitance value rapidly which would tune the tuner to a desired broadcast frequency.
Presently, a so-called frequency synthesizer tuner has been widely used, in which the local oscillating signal is generated by a phase-locked loop under the control of a digital frequency-selecting signal. In such a phase-locked loop, a variable frequency oscillator produces the local oscillating signal. In addition to being supplied to the usual mixer in the tuner, the local oscillating signal is supplied through a programmable frequency divider to a phase comparator where it is compared to a reference oscillating signal. Any phase difference therebetween results in an error signal which is fed back to the oscillator by tuning voltage, so as to adjust the frequency of the local oscillating signal and thereby adjust the tuning condition of the tuner. If the dividing ratio of the frequency divider changes, the frequency-divided oscillating signal which is supplied to the phase comparator will change. By well-known phase-locked loop operation, this changes the basic frequency of the local oscillating signal, resulting in cancelling the phase error signal.
In present television broadcast receiver the control of the tuning frequency is made via a microprocessor. The microprocessor send to the tuner on a bus various data bytes including for instance two bytes to settle the value of the divider and other bytes to control connection to be established in the tuner. Such tuner are generally made with a mixer-oscillator and phase lock loop integrated circuit. In addition to change in the divider value the control microprocessor must send additional information so that the first frequency-selective stages and outputs of the tuners will be settled according to the particular features of the tuner included in the television broadcast receiver. Such bytes are processed in the integrated circuit tuner through a bus interface. Each tuner of particular manufacturers, as explained above, is made with particular frequency band splitting so that according to the frequency chosen by the user, connection in the circuit will be made a particular way and outputs of the tuners will be available on one terminal or an other of the integrated circuit tuner. In spite of the fact that there is now a so-called xe2x80x9cWorld Standard Pining Frequency Synthesiser tunerxe2x80x9d, in which the size of the box and the connection pins are standardised, one integrated circuit tuner is not readily interchangeable with a circuit of an other manufacturer. To use alternative FST (Frequency Synthesiser Tuner) Tuner for the receiver chassis without any change in software tuning is not possible for most (all) of the application today. The software included in a microprocessor that controls the Tuner must include information like band partition of the tuner, reference divider, control byte of the tuner integrated circuit. Therefore, it is necessary in case of a change of the type of tuner, in particular if a tuner of another maker is used to change the software, and that is a serious drawback for the new incomer on the market of tuners.
The present invention seeks to provide improved tuning apparatus.
According to an aspect of the present invention, there is provided a tuner for receiving a video broadcast signal, the tuner comprising commandable circuits operable to select a tuned frequency, the tuner comprising a bus interface operable to receive data bytes and among those data bytes, bytes representing a divider ratio N and address bytes, wherein bytes received by the bus interface are processable through a N divider comparator operable to compare the value of divider ratio N received from the bus interface to values stored in a register, said comparator being operable to output control bytes and data bytes to be sent to the commandable circuit of the tuner to select a tuned frequency.
The preferred embodiment takes advantage of the fact that all tuners dedicated to a region have the same frequency ranges and the same intermediate frequency. So that the divider to get a desired frequency is the same for most of the tuners. What differs from one type of tuner to another is the frequency band partition in the RF stages. So according to the preferred embodiment, when existing software data from any of the already existing Frequency Synthesiser Tuner, is received at input of the bus transceiver with correct address bytes information, divider data byte 1 and 2 will be transferred to a N divider logic table comparator. It is to be noted that generally the divider ratio is given in two bytes, allowing for a divider ratio between 32767 and 256. The comparator inputs the divider data and outputs all other necessary data bytes taking in account the particular technical features of the tuner in which the comparator is included. The comparison will provide all necessary output information to complete the frequency synthesiser tuning functions:
a) Frequency bandswitch (usually greater than 2 Bandswitch) and output port information
b) Frequency N divider value
c) Self generated control byte information. (The external control byte information coming from the microprocessor of the receiver via an lC Bus is no more required.)
As such, the output ports and control byte coming from the microprocessor through an lC bus can be ignored.
There are several advantages using this xe2x80x9cUTSxe2x80x9d Frequency Synthesiser Tuner:
1) Fully compatible World Standard Frequency Synthesiser Tuner in pining and software is achieved, and provide xe2x80x9cplug-and-playxe2x80x9d possibility.
2) Great flexibility in tuner design to achieve best optimised performance in frequency band partitions and selection of integrated circuit without constraints from keeping tuning software compatibility.
3) Most of the existing or old tuning software can still be used with tuner equipped with this universal tuning synthesiser (xe2x80x9cUTSxe2x80x9d).
4) Enormous benefit to Tuner user in application, performance and cost.
5) Facilitate after sale servicing on FST (replacement or upgrading), just as convenient like a VST (voltage synthesizer Tuning) tuner.
6) Allow the feasible development of build-in standard FST tuning protocole into microprocessor (control of the instrument) directly as functional feature to control tuner without any additional written software for FST.